The Sporting defender, Marcos Rojo, played a key role in Argentina’s run to the final of the World Cup in Brazil this summer. Photograph: Mike Egerton/PA

Manchester United believe a deal to land Marcos Rojo is close, but it is still to be decided whether Nani should go on loan to Sporting Lisbon in addition to the £16m fee for the defender.

Club sources are calling the move for the Argentinian “very complex”. Yet once an agreement can be reached regarding whether Nani will move to Sporting in return – probably for a season – there is confidence at Old Trafford that Rojo will become a United player and so strengthen an area of Louis van Gaal’s squad that is particularly weak.

Given Van Gaal’s favoured 3-5-2 system, the Dutchman requires at least one left-sided defender. Phil Jones, Chris Smalling and Jonny Evans, his three recognised first-team players in the position, are all predominantly right-sided.

If Nani does go to Sporting it will mark a return to the club where he played for two seasons until his 2007 move to Old Trafford. Despite flashes of brilliance the Portugal winger has failed to consistently deliver at Manchester United.

Rojo apologised on Sunday after refusing to train in an effort to push through the move. “I was hot-headed and I’m sorry. I’ll be back with the rest of the squad tomorrow,” he said.

“Sporting signed me when I was not at my best and gave me the opportunity to play for a big club in Europe. I would like to thank the trust they had in me and I will resume training on Monday.”